1. Field of the Invention
The present invention relates to a sulfonium salt useful for a photo-acid-generator (i.e., a chemical agent for generating an acid through irradiation), which is readily decomposed through irradiation with an actinic ray such as a deep UV ray, an electron beam, an X-ray, or an EUV (extreme UV ray), particularly for a photo-acid-generator for use in chemically amplified photoresist material.
2. Description of the Related Art
In the field of semiconductor devices such as large-scale integrated circuit devices, typically DRAMs, there is keen demand for further enhancement in density, integration, and operational speed. In order to satisfy the demand, in industrial production of electronic devices, a more rigorous level of half-micron-scale microprocessing technique; e.g., photolithographic technique for micro-patterning, is required. In micro-patterning through photolithography, enhancement in resolution (R) is a key factor. The resolution can be enhanced by employing an actinic ray (exposure light) having a shorter wavelength λ in patterning the resist material, since resolution (R) of a demagnification stepper is calculated by Rayleigh's formula: R=k·λ/NA (wherein λ represents the wavelength of exposure light, NA represents numerical aperture, and k represents a process factor).
Regarding photoresists, U.S. Pat. No. 4,491,628 and other documents disclose chemically amplified photoresists as those suitable for exposure light of short wavelength. A characteristic feature of chemically amplified photoresist is that a photo-acid-generator contained in such a resist generates protonic acid through irradiation with light, and the generated protonic acid catalytically reacts with a resist resin or other components of the photoresist when the photoresist is heated after completion of exposure. Most photoresists currently in use are of the chemically amplified type.
Meanwhile, a variety of sulfonium salts are known to serve as a photo-acid-generator for chemically amplified photoresists. However, such conventional photo-acid-generators have drawbacks; for example, poor compatibility to a photoresist polymer having an acid-dissociable group; i.e., a group dissociated or decomposed by acid. Needless to say, when a photoresist containing such a photo-acid-generator is patternwise exposed to an actinic ray, the obtained pattern has unsatisfactory shape characteristics, which is problematic.